Urho3D/UI/ is legacy built-in Urho3D UI system. It is the most flexible UI solution, but it requires the biggest amount of hand-holding as well. You can do everything, but you will have to write a lot of code to make it work smoothly. Not actively maintained, bug fixes and minor tweaks only.
Urho3D/RmlUI/ is the integration of RmlUI library. Work in progress, but it is already useful. HTML/CSS like UI which requires certain knowledge to setup, but it supports data bindings and UI generation natively. DPI-aware. Check out RmlUI documentation.
Urho3D/SystemUI/ is the integration of Dear ImGUI library. Unlike other two UIs, it supports multiple native windows. It's not recommended to use it for actual game. Use this UI for Editor add-ons or other tools. Check out Dear ImGUI documentation.